Remove deprecated defaultPath migration helper from PageBlueprint

The `defaultPath` param was a compile-time migration artifact typed to
produce an error, guiding users to use `path` instead. The migration
period is over, so this removes it from the public API surface.

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
Made-with: Cursor

plugins/api-docs/report-alpha.api.md

@@ -563,7 +563,6 @@ const _default: OverridableFrontendPlugin<
       kind: 'page';
       name: undefined;
       params: {
-        defaultPath?: [Error: `Use the 'path' param instead`];
         path: string;
         title?: string;
         icon?: IconElement;